Sausage: host, broker, commit-on-close filesystem, cargo actor, console, agent

Imported from the spike branch, with:
- a library crate (Host API) for the CLI and the tests
- JSON test scenarios for candidates
- commit on close only for new or truncated files
- a Nix flake: nix run -- init, nix run -- run

examples/echo/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "echo"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+serde_json = "1"

examples/echo/src/main.rs

@@ -0,0 +1,15 @@
+use serde_json::{json, Value};
+use std::io::{BufRead, Write};
+
+fn main() {
+    let mut out = std::io::stdout().lock();
+    for line in std::io::stdin().lock().lines() {
+        let Ok(msg) = serde_json::from_str::<Value>(&line.unwrap()) else { continue };
+        if msg["body"]["type"] == "shutdown" {
+            break;
+        }
+        let env = json!({"to": msg["from"], "reply_to": msg["id"], "body": {"type": "echo", "got": msg["body"]}});
+        writeln!(out, "{env}").unwrap();
+        out.flush().unwrap();
+    }
+}